Radiotherapy equipment has been widely applied, wherein the grating blade device for fitting the shape of a lesion and defining a radiation field of rays has been greatly developed. The structural shape of blades, the arrangement of blades and the precision of fitting the radiation field are all continuously improved. However, in the prior art, the control on accurate positions of grating blades driven by a motor is difficult to master, and a motor driving system itself has natural defects, e.g., transmission error of the screw, instability of the motor, etc., so that the precision of fitting the radiation field cannot be further improved and the verification of the equipment is very complicated and difficult.